Embodiments of the present invention relate in general to radial stacking angle measurement for blades.
The term “stacking axis” should be known to the persons skilled in the art; anyway, for the sake of completeness, a definition is set out in the following. A rotating blade of a rotary machine, in particular a turbo machine, may be considered as the combination of a plurality of blade sections stacked one on top of the other in a radial direction that is perpendicular to the rotation axis. The line connecting the centers of gravity of each section defines the stacking axis.
Other elements of the blade are the leading edge that is the front part of the airfoil and the trailing edge that is the rear part of the airfoil. The blade extends radially outwardly from the root, at a first end close to the hub (often adjacent to the hub), to the shroud, at a second end remote from the hub. The radial axis is the axis outwardly extending from the center of the hub. The stacking axis of a so-called cylindrical blade should typically coincide with the radial axis. However, tolerance of the machines used for manufacturing of the blades and forcing of the blade during the process of assembly may cause an out-of-radiality stacking of a blade.
FIGS. 1A and 1B shows two different types of out-of-radiality stacking. In FIG. 1A, due to tolerances in manufacturing procedure of the shroud 4 of the blade 1, there is an excess of material of the shroud itself. This situation can occur, e.g., if the angle by which the shroud is manufactured is slightly greater than the project one. The radial axis is indicated with the reference R, whilst the stacking axis is indicated with the reference S. The two axes are misaligned, whilst, as said, should coincide. Therefore, in the case of FIG. 1A it is necessary to correct this defect, e.g. milling the shroud to give it the correct angle.
FIG. 1B shows a blade affected by an out-of-radiality stacking occurred by a wrong (due to tolerances) manufacture of the root 18. This condition creates a misalignment of the stacking axis S with respect to the radial axis R. It worth noting that in the case of FIG. 1B the misalignment of the two axes is opposite with respect to the case of FIG. 1A.
Consequently, centrifugal stresses, due to misalignments to which a blade is subject, can be substantial, producing considerable wear. The design of a blade tends to an increase in the life time. In machines having blades with radially directed straight stacking axis, if the radial parallelism is not correct, the design stresses of the blade can increase, and the safety coefficient can be lowered.
Therefore, it is important to measure the angular amount of the stacking in order to perform the necessary corrections. For this type of measurements, it is known to use only one simple radial ruler; anyway, such ruler can be used only with blades having one parallel-to-radial tangential side because the reference radial vector can be found and depicted on the blade. Furthermore, such ruler does not give a quantitative value of the out-of-radiality stacking, but rather only a qualitative estimation, since the measurement is performed only by eyes without the help of a measurement scale. Until today tools or methods for quantitative measurements of the angular amount of the stacking, when blades have no any parallel to radial tangential side, are not available.
Therefore, there is a need for providing a tool for measuring quantitatively radial stacking angle of blades, in particular for any shape of the blade.